1. Field of the Invention
The present invention relates to a method and system for controlling the microstructure of an optical fibre including a void-containing region. In particular, it relates to a method and system for monitoring the microstructure of an optical fibre along the fibre length during fibre drawing. According to an aspect, the present invention relates to a process for manufacturing a microstructured optical fibre.
2. Description of the Related Art
Holey or photonic crystal fibres have been studied in the last several years because of their properties that are rather different from the conventional fibres. Holey fibres are generally fabricated entirely of a single material, typically of bulk fused silica. The refractive index contrast between the core and the cladding of the optical fibre is achieved by incorporating a photonic crystal-like structure into the cladding. The pattern of holes, typically air-filled, leads to an effective lowering of the refractive index. Since only a small fraction of the transmitted light travels in the cladding, pure silica core fibres can potentially maintain the low loss of pure silica.
Holey or photonic crystal fibres can be manufactured in several different ways. One method, known as ‘stack-and-draw’, includes stacking silica capillary tubes inside a hollow glass cylinder in a close-packed space arrangement, welding the tubes together and then drawing the resulting preform by a conventional fibre preform drawing method.
U.S. Pat. No. 6,243,522 discloses a preform for making a photonic-crystal fibre, the preform having a core body surrounded by a clad layer formed by a plurality of clad rods. The clad rods have a central portion surrounded by a layer of larger refractive index. According to an embodiment, the preform is constructed by inserting clad rods, which are placed in a periodic array, and core rod into a hollow glass tube.
US patent application No. 2008/0138022 discloses a microstructured optical fibre made of a single, doped material matrix, preferably pure silica, having longitudinal holes forming two or three rings around the core, wherein the average distance between the holes is of at least 6 μM. The described fibre is said to be suitable for long-distance telecommunications, having a particularly reduced signal loss thanks to an optimised balance between Rayleigh scattering and radiation leakage through the cladding.
Very recently, a different class of microstructured optical fibres has been developed, the fibres including a solid central core surrounded by a hole-containing silica cladding, wherein the holes are arranged in a random or non-periodic spatial distribution.
WO patent application No. 2008/005233 discloses an optical fibre comprising a core region and a cladding region surrounding the core region, the cladding region comprising an annular hole-containing region comprised of non-periodically disposed holes. The core region and cladding region is said to provide improved bend resistance and single mode operation at wavelengths greater than or equal to 1500 nm, in some embodiments greater than 1260 nm. Preferred embodiments described in WO 2008/005233 disclose hole-containing regions that are spaced apart from the core of the optical fibre, but not extending entirely to the outer perimeter of the fibre.
WO patent application No. 2007/055881 describes a method of making a microstructured optical fibre comprising forming via chemical vapour deposition (CVD) operation a soot containing optical fibre preform. The soot preform is consolidated in a gaseous atmosphere which surrounds the preform under conditions which are effective to trap a portion of the gaseous atmosphere in the preform during said consolidation step, thereby resulting in the formation of non-periodically distributed holes or voids in the consolidated preform, each hole corresponding to a region of at least one trapped consolidated gas within the consolidated glass preform. At least some of the holes formed in the optical fibre preform during the consolidation step remain in the drawn optical fibre.
In generally known methods for making optical fibres, once the manufacturing of a preform is completed, the preform is lowered at a relatively low speed into a furnace having a hot zone in which the preform is melted at typical temperatures of 2000° C.-2200° C. so that the fibre lower end of the preform forms what is known as the neck-down region, where glass softens and is submitted to reduction in cross-sectional area to the desired cross-sectional area of the fibre. From the lower tip of this neck-down region, the optical fibre emerges where it can be gripped by a mechanical device.
Optical fibre technology requires characterization and control of various fibre properties during the process of drawing the fibre from a preform. In particular, fibre performance depends critically on the geometric uniformity and the dimensions of the core and cladding layers of the fibre. These fibre parameters are typically monitored during the drawing process without perturbing the fibre or the process. The outer diameter is generally measured at a point shortly after the fibre is formed (immediately below the neck-down region).
U.S. Pat. No. 3,982,816 discloses a method of measuring the outer diameter of an optical fibre by using a beam of coherent monochromatic radiation directed to the fibre to generate a far-field scattering pattern. A portion of the far-field scattering pattern results from radiation reflected from the outer surface of the fibre and radiation passing through the fibre and being refracted predominantly by the outer cladding. The number of fringes is counted between a lower scattering angle and an upper scattering angle in the particular portion of the scattering pattern. The outer diameter of the fibre is then calculated from the number of fringes.
U.S. Pat. No. 4,280,827 describes a fibre diameter measurement circuit including a source, a detector that senses the presence of interference fringes, wherein the detector signal is connected to two signal comparing means via a respective delay circuit connected to the source. The outputs from the signal comparing means are combined and counted in order to generate a succession of counts representative of the diameter of successive axial portions of the advancing fibre.
When a fibre is being drawn at or near its target diameter, the location of each fringe is predictable. Using this fact, holes have been detected by watching for a missing sequence of fringes of a prescribed user-settable length.
A method and apparatus for detecting defects in optical fibres based on different parameters from those used to measure fibre diameter is described in U.S. Pat. No. 5,185,636. A disclosed technique involves generating a spatial frequency spectrum for the detected pattern (i.e., the Fast Fourier Transform), which contains a line component to the outer diameter of the fibre. When the fibre contains a defect, the spectrum will contain a second component whose frequency (or frequencies, when split) is less than that of the line component corresponding to the outer diameter.
In U.S. Pat. No. 6,313,909 a scattered light signal is filtered and the resulting signal is compared to a defect detection threshold to determine the presence of defect-related components in the scattered light signal. A filter removes first and second components of the scattered light signal to generate a modified scattered light signal, wherein the first component corresponds to the fibre diameter measurement system and the second component corresponds to the outer diameter of the fibre; a defect sensitivity adjuster provides a defect detection threshold, wherein the defect detection threshold corresponds to a portion of a reference signal; and a comparator compares the modified scattered light signal to the detection threshold to determine if the defect-related component is present, the presence of the defect-related component being indicative of the presence of a defect in the fibre.
A detection device for determining defects in a fibre is disclosed in US patent application No. 2003/0231296. The device includes three bandpass filters: a regular airline filter, a clad diameter filter and a core airline filter. Scattered light is passed through the filters to generate three respective signals. The regular airline and clad diameter signals are compared to produce a normalised regular airline signal; the core airline signal and clad diameter signals are compared to produce a normalised core airline signal. A change in the strength of the normalised regular airline signal indicates the presence of a defect in the overclad region and change in the strength of the normalised core airline signal indicates the presence of a defect in the core region.
As described for instance in U.S. Pat. No. 3,982,816, on-line optical fibre inspection based on the analysis of light interference patterns in the far field produced by the light reflected and refracted from an optical fibre transversely illuminated by monochromatic light provide a direct relationship between the outer diameter of the fibre and the fringe count, when the refractive index of the fibre, and in particular of the fibre cladding, is substantially constant. In that case, the number of fringes counted in the interference pattern across a given angular range multiplied by an empirical conversion factor, which is mainly dependent on the setting parameters of the measurement system, provides the outer diameter.
Applicant has observed the following. Measurement methods as that described in U.S. Pat. No. 5,185,636, when used for on-line monitoring during fibre drawing, are typically set to detect structural defects, such as voids, as an anomaly in the fibre structure. Since microstructured optical fibres include deliberately-introduced defects, which generally are voids running longitudinally along the fibre axis, methods employing counting of the fringes of the as-detected far-field interference patterns seem not to be suitable to monitor the outer diameter of a microstructured fibre.
Optical fibres having a low-density region with a non-periodic, and in general random, distribution of voids (i.e., voids are irregular in their location within the region), the low-density region being disposed around the fibre core, and preferably in the fibre outer cladding, may be tailored to provide single-mode transmission and robust bend-resistance. Herein, the term void may indicate empty holes, air-filled holes or bubbles containing gases trapped within them, and in general a defect having a refracting index significantly smaller than that of the surrounding matrix, and generally having a refractive index equal or close to 1.
An optical fibre including a low-density region with a random void distribution can be advantageously manufactured during formation of the preform by a sintering process in which gases with low-solubility in the materials forming the fibre, usually silica-based materials, remain trapped and form voids. The preform can be manufactured in two main steps: first, a glass core rod including the preform core, which is preferably void-free, is produced by deposition and then consolidated, and, second, a preform outer cladding is formed around the glass core rod by deposition and then consolidated to form voids within the preform outer cladding. The resulting consolidated preform typically exhibit an annular low-density region including a random distribution of voids, which starts at about the interface between the core rod and the outer cladding and extends radially within the outer cladding for a certain thickness. Thickness of the low-density annular region, hereafter also referred to as the void-containing ring, and local void density within the ring may widely vary in dependence on the sintering process conditions, such as consolidation time, temperature gradient in the furnace and percentage of volume of low-solubility gases during consolidation.
The drawing process following the formation of the preform, in which the preform glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fibre, inevitably have an effect on the voids, the main one expected to occur being a stretching of the voids along the longitudinal axis of the drawn fibre.